In many applications it is desirable to store data during memory device manufacturing in order to later utilize that preprogrammed data in a system in which the memory device is included. For example, preprogrammed data can be stored in nonvolatile memory devices during manufacturing, where the preprogrammed data is later retrieved by a processor or other entity in a system that includes the nonvolatile memory devices and used for operations such as startup or initialization. Similarly, in single-chip environments, preprogrammed data can be stored in nonvolatile memory embedded on the chip and then later used by logic circuitry or other circuitry on the chip when the chip is later included in a system.
Preprogramming data in nonvolatile memory during manufacturing is relatively straightforward as the conditions are present to enable such programming during testing operations such as wafer probe or burn-in. However, such preprogrammed data can be put in jeopardy during subsequent manufacturing operations. For example, soldering a finished integrated circuit onto a printed circuit board using reflow soldering techniques can expose the integrated circuit to temperatures on the order of 260° C. Such elevated temperatures can cause degradation of stored memory states, thereby potentially corrupting the preprogrammed data stored earlier in the manufacturing process.
For example, in the context of magnetoresistive memory devices, exposure to such elevated temperatures can degrade the magnetic states used to store the preprogrammed data, thereby resulting in undesirable data loss. Magnetoresistive memory devices store information with magnetic states that result in different device resistances. For example, in certain magnetoresistive memory devices, the resistance across a magnetic tunnel junction (MTJ), and therefore the voltage drop for a specific current, depends on the relative magnetic states of the magnetic layers within the memory cell. In such memory devices, there is typically a portion of the memory cell that has a “reference” magnetic state and another portion that has a “free” magnetic state that is controlled to be either parallel or antiparallel to the reference magnetic state. Because the resistance through the memory cell changes based on whether the magnetic state of the free portion is parallel or antiparallel to the magnetic state of the reference portion, information can be stored by setting the magnetic orientation of the free portion. The information is later retrieved by sensing the orientation of the free portion. Such magnetic memory devices are well known in the art.
Similarly, other memory devices exist that store data in ways that can be compromised by exposure to heat or other adverse conditions presented by the manufacturing processes. For example, other forms of resistive memory store data based on the state of the materials included within the memory device, where the state of those materials can be impacted by exposure to the heat associated with reflow soldering. One example includes state change memory in which data is stored based on whether a layer of material is in an amorphous state. While the effects of such exposure to heat may not cause total loss of data in the memory affected, some subset of the bits included in the memory can be compromised, thereby resulting in undesirable errors in the overall data set.
As such, there is a need for techniques to allow preprogrammed data to the stored in such memory devices and later reliably recovered after the memory is exposed to adverse conditions, including the heat associated with soldering operations and/or packaging.